An Efficient Process in the Production of Iron and Steel from Iron Ore

ABSTRACT

The invention discloses a method of reducing iron oxide to recover iron from iron content mixture. The method includes the step of mixing a reductant containing at least one of a silicon carbide or ferro-silicon with iron content mixture and heating it at a temperature ranging from 600-1000° C. for reaction to undergo. The iron content mixture is one of an iron ore, iron oxide and gangue material. The addition of reductant during the manufacture of sponge iron, DRI or pig iron or during reduction of iron ore assures advantages such as better converts yield, efficient use of resources, thus resulting in increasing productivity and reducing the cost of manufacturing. The method converts and upgrades low grade iron ore to high grade iron ore by dry beneficiation magnetic separation. The method also helps in recovery of iron from slag generated during iron and steel production.

FIELD OF THE INVENTION

The present invention relates to a method for reducing iron oxide to recover iron from iron content mixtures or iron ore. The method discloses an efficient way of reducing more iron oxide than from the conventional reducing methods and also reduces the volume of wastage generated in the process of iron and steel production.

BACKGROUND OF THE INVENTION

In the existing technology of manufacturing iron or steel using Direct Reduced Iron (DRI), or sponge iron, typically there is around 15-20% slag is generated which mainly consists of unreduced or un-reacted iron oxide and gangue materials in sponge iron or DRI. This unreduced iron oxide and gangue materials is discarded as slag upon subsequent melting in electric induction or electric arc furnace during iron and steel production. Presently an additional energy consuming process is adopted to re-melt and separate slag from sponge iron, DRI or pig iron. So preparing the raw materials (feed-stock) is a crucial step in the manufacture of iron and steel.

Sponge iron is produced from Hematite or Magnetite iron ore or both or from iron oxide pellets by removing the combined oxygen through direct reduction process using non coking coal as fuel and reducing agent in rotary kiln. In case of pig iron production, iron ore pellets are subjected to reduction, by use of metallurgical coke and preheated hot air in a blast furnace.

Various rotary kiln processes for production of sponge iron are available such as SL/RN process, Krupp Codir process, SIIL process etc. But the various rotary kiln process as discussed above has the tendency for formation of clusters, agglomerates and accretion or ring formation during actual manufacture of sponge iron in the temperature range of 900° C. to 1100° C.

There have been prolonged attempts to manufacture sponge iron and semi-fused iron at higher temperature by using various reactors like gas, solid reactor, moving bed reactor, horizontal rotary kiln reactor etc. Sponge iron is also manufactured in various parts of the world through either natural gas or coal-based technology. Iron ore is reduced in solid state at 800 to 1,050° C. (1,472 to 1,922° F.) either by reducing gas (H₂+CO) or coal.

For instance, the European Patent No. EP1187941 (A1) discloses a method for manufacturing granular metallic iron that includes heating of a formed raw material comprising a carbonaceous reductant and a substance containing iron oxide in a reduction melting furnace to subject the iron oxide contained in the formed raw material to solid-state reduction and carburizing reduced iron resulting from the solid-state reduction with carbon contained in the carbonaceous reductant to cause the reduced iron to melt, while separating off gangue components contained in the formed raw material and causing resulting molten metallic iron to coalesce into the granular metallic iron, wherein an atmospheric gas present in proximity to the formed raw material in the carburizing and melting step has a reduction degree of not less than 0.5. The invention is also directed to a method of producing metallic iron, including forming a deposit layer containing slag produced in the reduction melting process on hearth refractories, thereby protecting the hearth refractories while producing the metallic iron. The invention is further directed to a device for supplying an auxiliary raw material to a hearth of a moving hearth type reduction melting furnace adapted to produce metallic iron, the device including a supply duct vertically connecting with a ceiling portion of the furnace.

Another European Patent No. EP0128347 (A2) discloses a method for the gaseous reduction of iron ore to sponge iron in a vertical shaft moving bed reactor wherein a hot gaseous mixture of hydrogen and a carbon monoxide is used to reduce the ore. It has been found unnecessary to use the external catalytic reformer of prior art processes to reform the methane and other hydrocarbon constituents of the natural gas commonly used as a source of gaseous reductants. By establishing a reducing gas recycle loop to which make-up natural gas and steam are fed in the proper proportions, the product sponge iron in the reactor can be used to catalyze hydrocarbon reformation, provided that the carbon content of the recycled gas is maintained low by removing carbon dioxide there from. When pure methane is employed, steam/methane molar ratios within the preferred range of 1, 4:1 to 2.2:1 are used.

U.S. Pat. No. 4,216,011(A) discloses a method and apparatus for achieving improved reduction efficiency and thermal economy in the reduction of particulate metal ores, e.g., iron ore, in a vertical shaft, moving bed reactor. The reactor consists of a primary reduction zone, a cooling zone and a secondary reduction zone located between the primary reduction zone and the cooling zone wherein ferric carbide formed in the primary reduction zone further reduces the residual iron oxide in the presence of an inert gas at a temperature below the melting point of the sponge iron. In one embodiment of the invention the gas produced by the reaction in the secondary reduction zone is withdrawn into a combustion chamber where it is mixed with air and burned, carbon dioxide is removed from the effluent gas of the combustion chamber and the remaining inert gas is re-circulated to the secondary reduction zone. In another embodiment of the invention inert gas is supplied from an external source and carbon dioxide is eliminated from the system by means of a controlled purge. Use of the disclosed secondary reduction system allows for a lower residence time of the metal ore through the reactor and increased thermal economy.

Another U.S. Pat. No. 4,416,688(A) discloses the process for beneficiating iron ore, particularly high phosphorous iron ore, by controlled selective solid-state reduction in which the resultant sponge iron is produced in a malleable condition followed by ultrafine preferential grinding to form flake powder of the sponge iron and produce finely divided oxide gangue, including the phosphate bearing materials, and separation of said flake powder and finely divided gangue by conventional concentration techniques to obtain a high iron flake powder concentrate.

Direct Reduction (DR) of iron ore to manufacture sponge iron or DRI (Direct Reduced Iron) by reducing agent or reducing gas has been developed to overcome some of difficulties of conventional blast furnaces. The direct reduction process is intrinsically more energy efficient than the blast furnace because it operates at a lower temperature and there are several other factors which make it economical.

Pig iron is an Iron ‘Fe’ based alloy generally having a carbon content of 3-5%, containing other impurities and produced by a metallurgical process from sintered, pelletized, or lump iron ores. Slagging material (limestone), metallurgical coke are added to sintered, pelletized, or lump iron ores in blast furnace. This mixture of input charge is burnt by preheated air (1000-1150 degree C.) blown into the blast furnace, increases the temperature and assists the coke in the reduction process to produce pig iron in blast furnace.

In the case of DRI or sponge iron production, the iron ore pellets are charged into the rotary or vertical shaft furnace and reduction of iron ore by carbonaceous material or gas is used. By using Pig iron or Sponge Iron or DRI, the secondary steel is produced in EAF or in Electric Induction furnace.

Reference from the “REPORT OF THE WORKING GROUP ON STEEL INDUSTRY FOR THE TWELFTH FIVE YEAR PLAN (2012-2017), Ministry of Steel, Govt. of India.

As per the report “During the iron and steel making process, the impurities present in the raw materials like iron ore, lime stone and coal are normally removed as slag. Further, the operations of air and water pollution control equipment generate dusts and sludge. Currently this volume of solid wastes generated in Indian steel plants is relatively high at 600-800 kg per ton of steel as compared to 400-500 kg in developed countries. This is mainly due to higher impurity levels in the raw materials. Large quantity of slag is produced in Basic Oxygen Furnace (BOF)/Electric Arc Furnace (EAF). It is not easy to dispose of the steel making slag due to the presence of free lime and high percentage of iron oxide”.

“Further the report says “Because of the specific nature of the feed materials like high gangue ore and high ash coke, Indian steel plants end up with high volume of slag resulting in poor productivity and higher energy consumption. Techno-commercial solutions for the raw material related issues are the need of the hour. The undesirable gangue from ore/coal needs to be removed as early as possible in the overall process. This calls for extensive beneficiation of Indian iron ore and coal by developing relevant technologies in-house through R&D intervention.”

“Steel making results in slag generation of about 180-200 kg/TCS in Indian plants. While a small portion of BOF slag is recycled through sintering operation, most of it is used for land filling. But neither of these processes ensures the most effective utilization of BOF slag. The disposal of EAF slag, likewise, is equally difficult.”

Iron ores are rocks and minerals from which metallic iron can be economically extracted. Ores carrying very high quantities of hematite or magnetite (greater than ˜60% iron) are known as “natural ore” or “direct shipping ore”, meaning they can be fed directly into iron-making blast furnaces. Most reserves of such ore have now been depleted. Iron ore is the raw material used to make pig iron, which is one of the main raw materials to make steel. 98% of the mined iron ore is used to make steel. With the huge growing demand for iron and steel products, the availability of basic raw materials such as iron ore reserves are getting depleted, causing environmental imbalance from large scale mining. As a result, many steel plants are looking at low grade iron ore (lesser than 40% Fe), to meet their demand, requirement and timely availability of raw materials. As such low grade iron ore cannot be used in iron and steel production, in order to be techno-economical, will have to be beneficiated to separate iron oxide from gangue materials (Silica, Alumina and other oxides). However with low grade iron ores, the beneficiation process is crumble some, leading to high process cost, high wastages (in the spent ore) in addition to additional water requirement which is very crucial.

Thus we need a method which provides maximum utilization of iron from low grade iron-ore, a method which increases the metallic iron yield in production of sponge/DRI/Pig iron, and a method to effectively recover iron from slag generated during iron and steel production.

SUMMARY OF THE INVENTION

According to an embodiment, the invention discloses a method of reducing iron oxide to recover iron from iron content mixture. The method includes step of mixing a reductant with iron content mixture. The reductant has at least one of a silicon carbide or ferro-silicon. The method also includes step of heating the iron content mixture at a temperature varying from 600° C. to 1000° C. for reducing iron from the iron content mixture. The method further includes as step of heating the mixture of reductant and iron content mixture by adding a fuel, wherein the fuel includes at least one of a metallurgical coke, coal or any other form of energy used for reduction to undergo. The iron content mixture is one of iron ore, iron oxide and gangue materials.

According to another embodiment, when the iron content mixture is of low grade iron ore then the method further includes step of dry beneficiation processing magnetic separation, wherein the dry beneficiation processing does not require water, which makes the process less complex, and very efficient and economical.

The present method provides higher yield of metallic iron in sponge iron/DRI. The present method also helps in recover useful iron from slag generated during iron and steel production. The present method also has the ability to convert low grade non-magnetic (hematite) iron ore to magnetic (magnetite) iron ore enables dry beneficiation method to upgrade into high grade iron ore through dry magnetic separation technique, thereby not consuming any water. The converted iron ore is much softer, thereby the energy required to further process the iron ore; i.e., wear and tear of tool and equipment during crushing is minimized. The method reduces the trailing loss in spent ore, when compared to wet beneficiation. Thus conservation of iron ore resource is possible. Iron and steel production becomes economical, as the dependency on high grade iron ore is not a criteria. The present method also helps in recovery of iron from the waste that is slag produced during iron and steel production, steel making in EAF, EOF, Induction furnace or LD convertor

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a table regarding sponge iron charge in induction furnace melting.

FIG. 2 illustrates the varying percentage of reductant added to high grade iron ore and its result.

FIG. 3 illustrates the content of low grade iron ore Sample 1.

FIG. 4 illustrates the converted high grade iron ore retrieved after adding the reductant to low grade iron ore.

FIG. 5 illustrates the reaction of slag with the reductant and the composition of the slag thus obtained.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise defined, all technical terms used herein have the meaning commonly understood by a person skilled in the art to which this invention belongs. One skilled in the art will recognize many process and materials similar or equivalent to those described herein, which could be used in the practice of this invention. Indeed, this invention is in no way limited to the process and a material described, but also applies to the extension and derivatives possible and originating from this invention.

According to an embodiment, the present invention discloses a method of reducing iron oxide to recover iron from iron content mixture. The method includes step of mixing a reductant with the iron content mixture. The reductant has at least one of a silicon carbide or ferro-silicon. The reductant has either silion carbide alone or ferro-silicon alone or it is a combination of both silicon carbide and ferro silicon. The method also includes step of heating the mixture of the reductant and the iron content mixture at a temperature varying from 600° C. to around 1000° C. for recovery of iron by reducing iron oxide from the iron content mixture. The iron content mixture is one of the iron ore or iron oxide or slag produced during reduction of iron ore or iron oxide.

The method for reducing iron oxide to recover iron from iron content mixture further includes step of adding fuel for heating the iron content mixture. The fuel is at least one of metallurgical coke or coal. The fuel may be any form of energy that may be used to heat the iron content mixture. The reduction of iron ore is achieved when the reaction temperature is attained i.e., around 600 to 1000 degree C.

According to an embodiment, the reductant has the following percentage of silicon carbide and ferro silicon along with lime. Silicon carbide is present in 92.5%, ferro silicon is 2.5% and lime is 5%. It is possible to use all grades of silicon carbide (SiC), ferro-silicon (FeSi) and lime. However the results provided by the reductant differ depending on the purity or grades being used. If the percentage of silicon carbide is reduced then proportionately ferro silicon percentage is increased and vice-versa. It is also noted that the above combination is best arrived formulation. However even with 95% silicon carbide and 5% lime or with 95% ferro silicon and 5% lime, reduction of iron oxide is obtained.

The present invention helps in reduction of iron oxide to recover iron during manufacture of iron or steel from iron ore or iron ore pellets by removing the combined oxygen, and by mixing the reductant which has at least one of a silicon carbide or ferro-silicon. The reductant has either silicon carbide alone or ferro-silicon alone or it is a combination of both silicon carbide and ferro silicon in combination with either gas base or coal/coke base fuel. The sponge iron, DRI or pig iron manufactured using the process of the present invention has better yield of metallic iron and results in effective utilization of energy and other resources. The resulting iron obtained using the present method has a number of applications in iron and steel production.

According to an embodiment, the present invention also discloses a method for manufacturing sponge iron or DRI or pig iron by reduction of iron ore, in which the melting time of sponge iron or DRI or pig iron is reduced in the electric induction furnace or Electric Arc Furnace (EAF) during manufacture of iron and steel. The method uses same/similar input resources as that of the existing technology for manufacturing iron ore pellets, but by the addition of the reductant which has either silicon carbide or ferro-silicon or combination of both to the iron ore results in higher output than the existing technique. Furthermore, the process for manufacturing iron ore pellets in accordance to the aspect of the present invention leads to better utilization of available natural resources and lesser environmental pollution as the amount of wastage (slag) generated in the process is reduced. Furthermore, sponge iron, DRI or pig iron leads to increased yield of metallic iron content and aims at effective utilization of available energy and resources towards increasing yield and reducing wastages.

According to an embodiment of the invention, the present method is used to produce sponge iron or DRI or for the manufacture of pig iron in blast furnace which is used in the reduction of iron oxide to recover iron. The method includes the step of mixing the reductant, which has either silicon carbide or ferro-silicon or both combined to the iron ore fines (the content of reductant is small percentage; in the range oft-3% by weight of the iron ore fines), limestone, bentonite or dextrin or any other binder and/or carbonaceous material to prepare pellets of iron ore. Iron ore pellets are used as a feedstock to manufacture sponge iron/direct reduction iron in rotary furnace/kilns and also in manufacture of pig iron in blast furnace.

Iron ore pellets are tiny spheres or green pellets that are constructed by combining iron ore with other products in order to create materials that are easy to transport. The range of additional ingredients used in the pellets may vary, although it is not unusual for each pellet to contain some type of clay or limestone, as well as elements such as dolomite stone and olivine as part of the mix. Bentonite is also sometimes added as a binding agent for the iron ore pellets, allowing the product to remain stable during transport and storage. As the name implies, iron ore pellets are small pellets that bear some resemblance to small pieces of rock. The pellets may be somewhat round or formulated into an elongated shape that resembles a very small piece of tubing. Creation of the pellets involves a process that is known as pelletizing, and calls for combining the iron ore with other ingredients in machinery that is referred to as pelletizers. The machines aid in shaping the raw material into small pieces that are more or less rounded. From there, the pieces are placed into a kiln and fired until they are firm and solid.

FIG. 1 illustrates the composition of sponge iron charge in induction furnace melting. The induction melting charge composition consists of 50% sponge iron and 50% steel scrap, generates 26.13% FeO. This 26.13% of FeO is nothing but un-reduced iron oxide. Whereas after adding the reductant containing silicon carbide and/or ferro silicon to the same composition charge i.e., 50% sponge iron and 50% steel scrap, the FeO generated is reduced to only 13.5%. The percentage of un-reduced FeO reduces from 26.13% to 13.5%. When charge composition is 25% sponge iron and 75% steel scrap, the un-reduced FeO percentage is 20.5% (without the addition of reductant). By the addition of reductant the percentage of un-reduced FeO generated is reduced to 15.2%.

Laboratory Illustration:

Typical composition of the iron ore is as follows. Fe is 64.35%, SiO₂ is 4.21%, Al₂O₃ is 1.9%, S is 0.02% and P is 0.05%. First step involves making pellets by mixing reductant containing silicon carbide and/or ferro-silicon (4 to 25%), 2% bentonite, 2% dextrine and 6% moisture. After mixing the pellets are sun dried for 2 or 3 days in-order to completely remove moisture. For example 200 gm pellets are put into a stainless steel cups and the cup was covered with ceramic lid. A small weight of 250 grams was placed on the lid top and the cup is placed inside the preheated furnace at 800 degree C. for 45 minutes and removed the cup from furnace and cooled to room temperature without removing the lid.

The reductant which has silicon carbide and ferro-silicon dissociates into individual elements. The reduction of iron oxide takes place as illustrated below. The reaction is an exothermic making the overall process energy efficient.

Iron oxide reduction by carbon and silicon is as follows:

3Fe₂O₃+C=2Fe₃O₄+CO

Fe₃O₄+C=3FeO+CO

FeO+C=Fe+CO and

3Fe₂O₃+½Si=2Fe₃O₄+½SiO₂

Fe₃O₄+½Si=3FeO+½SiO₂

2FeO+Si=2Fe+SiO₂

In another scenario, the reductant which is either silicon carbide or ferro-silicon or both is mixed along with the iron ore and uniformly mixed across the cross section of the pellet, the reaction takes place simultaneously generating heat and thus at short duration the metallization reaction completes and delivers high percentage of metallization and high grade sponge iron or DRI.

FIG. 2 illustrates the varying percentage of reductant silicon carbide and/or ferro-silicon mixture addition and its result.

According to an embodiment of the invention, the present method is also used to convert low grade iron ore (<40% Fe) into high grade iron ore. As such low grade iron ore is not suitable to be used in iron and steel production without beneficiation to high grade iron ore. In order to be successfully (economically) used in iron and steel production, iron oxide should be separated from gangue materials (Silica, Alumina and other oxides). Low grade ores are non-magnetic and is very difficult to beneficiate due its non-magnetic nature. In order to convert low grade non-magnetic iron ore into high grade iron ore, mix (3-4%) of reductant which is either of silicon carbide and/or ferro-silicon to the low grade ore of size below 5 mm, in a refractory pot and cover with a lid and pass it along a heated conveyor system at above 800 deg C, for about 45 minutes. At the end of the heating cycle, low grade non-magnetic ore is converted into magnetic ore. The gangue and non-magnetic impurities are separated by passing through the dry magnetic separator and the ore is upgraded and beneficiated. Converted ore which is further subjected to the existing process, of making sponge iron/DRI or pig iron.

FIG. 3 illustrates the content of low grade iron ore in Sample 1. Percentage of iron Fe is 26.58 where as percentage of SiO2 is 60.47, aluminium oxide is 0.61% and LOI is 0.1%.

FIG. 4 illustrates the upgraded high grade iron ore retrieved after adding the reductant which is silicon carbide and/or ferro-silicon. When the sample 1 is treated as stated that is crushing and separation the amount of magnetic Fe is 59.12% where as FeO is 57.44% and SiO₂ is 20.86%. When the sample 1 is treated with crushing and separation then the non magnetic Fe is 6.03% where as FeO is 4.75% and SiO2 is 90.80%.

According to an embodiment, the method disclosed is used for the recovery of iron from slag generated during iron and steel production. Slag generated is around 20% of the liquid metal (for example: 100 MT of steel production generates approximately 20 MT of slag). Out of this 20 MT of slag, the iron oxide content in it is around 5 MT (˜25% of slag). From this 5 MT of slag, roughly around 2-3 MT of useful iron is recovered by adding the reductant which is silicon carbide and/or ferro silicon in combination or each alone.

FIG. 5 illustrates the reaction of slag with the reductant and the composition of the slag thus obtained. In trial 1 about 200 kg of mixture containing reductant which is silicon carbide and/or ferro silicon is mixed to the slag and treated then the amount of un-reduced FeO is reduced from 22.60% to 21.90%. In another trial when about 225 kg of mixture containing reductant which is silicon carbide and/or ferro silicon is mixed to the slag and treated then the amount of un-reacted FeO reduced from 22.68% to 19.59%.

After tapping of liquid metal, the slag that is generated is treated with an addition of the reductant which has silicon carbide and/or ferro silicon mixture (around 3-4% by weight of the liquid slag), which when comes in contact with the hot liquid slag, the reductant reacts with the oxides in the slag; thereby reducing it and in-turn help in recovery of iron, which is otherwise discarded.

The present method provides higher yield of metallic iron in sponge iron or DRI. This method helps in higher output with the same input resources than the existing methods. Melting time of sponge iron or DRI in electric induction or arc furnace is reduced. The slag (wastage) generated is reduced. The reductant which is mixture of either silicon carbide or ferro-silicon or both act as a reducing agent and accelerates the reduction process of iron oxide.

The present method also has the ability to convert low grade non-magnetic iron ore to magnetic iron ore and enables dry beneficiation to high grade iron ore through dry beneficiation technique, a technique in which water is not required for processing the low grade iron ore. In normal wet beneficiation water is the biggest consumable, which also involves extremely large scale of equipment and complex process steps when dealing with millions of tons of ore. The converted ore using the present method is much softer, thereby the energy required and wear and tear of tool and equipment during crushing is minimized. The method reduces the trailing loss in spent ore, when compared to wet beneficiation. Thus conservation of iron ore resource is possible. Steel production becomes economical, as the dependency on high grade iron ore is not a criteria.

The present method also helps in recovery of iron from the waste that is slag produced during iron ore reduction. There is no energy requirement and no additional time required for the same. This also reduces the landfill mass and helps in effective utilization reclamation of raw material 

I claim:
 1. A method of reducing iron oxide to recover iron from iron content mixture, the method comprises step of: a. mixing a reductant with iron content mixture where in the reductant has at least one of a silicon carbide or ferro-silicon; and b. heating the mixture of the reductant and the iron content mixture at a temperature varying from 600° C. to 1000° C. for the reaction to undergo.
 2. The method as claimed in claim 1 further comprises step of: adding a fuel for heating the mixture of reductant and iron content mixture, wherein the fuel includes at least one of a metallurgical coke, coal or any other form of energy used for reduction to-undergo.
 3. The method as claimed in claim 2, wherein the iron content mixture is one of an iron ore, iron oxide and unwanted gangue materials.
 4. The method as claimed in claim 3 wherein the iron content mixture is of low grade iron ore then the method further comprises step of: dry beneficiation processing magnetic separation, wherein the dry beneficiation processing doesn't need water, which is very efficient and economical, without water requirement. 